A fuel tank of a gasoline-powered passenger car typically includes a fuel level sensor and a fuel tank pressure sensor. The fuel level sensor measures the amount of fuel remaining in the fuel tank so that the driver is alerted when the fuel level is low. By monitoring the fuel level measurements, the driver can refuel, as necessary, and can avoid the inconvenience of running out of fuel while driving.
The fuel tank pressure sensor measures the fuel tank pressure with respect to the atmosphere and detects leaks in the fuel venting system. In the United States, leak detection devices are currently required under the Environmental Protection Agency's “OBD II” requirements. Although leak detection algorithms can vary, a typical leak detector system closes the vent and then measures the tank pressure for a number of hours after the engine is shut off. Because the system is sealed and the engine has been shut off for some time, the temperature drops and the fuel vapor contracts, creating a partial vacuum. If the sensor measures a negative pressure relative to the atmosphere, it determines that the vent system is leak-free.
A typical method of measuring fluid level is to utilize a sensor for measuring the pressure of the fluid column. These types of sensors are generally used in connection with household appliances, such as washing machines. In a typical implementation, the sensor is placed at the top of the appliance and an open-ended tube is positioned in the tank (or tub, in the case of a washing machine). When the tank is filled, the resulting pressure in the tube containing air is measured. The pressure in the tube is proportional to the fluid level and therefore, the fluid level can be ascertained. A disadvantage of such a sensor is that if the tube is immersed in liquid for an extended period of time, the air in the tube eventually dissolves in the surrounding liquid. The liquid enters the tube and any resulting pressure measurement is compromised. Given the limitation of such a sensor, it is only a viable option for appliances such as washing machines, were the sensor is immersed in the water for a relatively short period of time during each cycle, and thereafter the water drains from the washing machine tub. However, in a fuel tank application, ideally, the fuel tank would not be cyclically emptied like a washing machine. Because the sensor would be immersed in gasoline continuously, the pressure reading could be compromised when air in the tube dissolved in the gasoline and the gasoline entered the tube. Thus, the sensor would be accurate only when the fuel level is close to empty such that the fluid would not enter and rise up in the tube.
Other types of sensors can be used but also have disadvantages. For example, one known sensor, with a movable arm and float, is inserted into the fuel tank through a small hole. Frequently, the sensor is bent and damaged during installation. Furthermore, even if the sensor is threaded through the hole without damage, if the float hits the bottom of the tank, it may stick there, rendering it useless.
Accordingly, it is desirable to provide a sensor that is easy to install, provides accurate readings and has minimal risk of damage during installation and use.
In the past, two separate sensors have been used, one to measure the fuel level and the other to detect leaks. It would be desirable to reduce cost and improve the reliability of the system by providing a single sensor that could both measure the fuel level and detect leaks.